85 research outputs found
Measuring cosmological weak lensing using the Advanced Camera for Surveys on board the Hubble Space Telescope
Following from the theory of General Relativity, light-bundles are deflected and differentially distorted while passing through the gravitational potential of matter inhomogeneities. The gravitational lensing effect caused by the large-scale matter distribution in the Universe is termed cosmological weak lensing, and provides a powerful probe of cosmology. By studying the distortions which are imprinted onto the observed shapes of distant galaxies, the statistical properties of the foreground density field can be constrained free of assumptions on the relation between luminous and dark matter. Due to the weakness of the effect, it is challenging to measure and can only be detected statistically from large ensembles of coherently lensed galaxies. In addition, careful correction for systematic effects is required, first of all for the image point-spread-function (PSF). In this PhD thesis we present a detailed cosmological weak lensing analysis using deep high-resolution images from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). Including data from the ACS Parallel Cosmic Shear Survey, the HST/GEMS Survey, and the HST/COSMOS Survey, this data set constitutes the largest survey used to measure cosmological weak lensing from space today. In order to achieve the high accuracy required for weak lensing studies, we developed several upgrades for the data reduction pipeline including careful image registration, improved bad pixel masks, and an optimised weighting scheme. We also perform a thorough investigation of the ACS PSF and develop a new correction scheme for its spatial and temporal variations, which are caused by thermal breathing of the telescope. We present numerous tests of our shear measurement pipeline using simulated images from the STEP Programme, and demonstrate that it achieves a relative shear-measurement accuracy better than 2% for ACS-like images. We perform the analysis of the ACS data in two steps, starting with a pilot study, in which we test the capabilities of ACS for cosmological weak lensing measurements with early parallel observations and the combined GEMS and GOODS ACS mosaic of the Chandra Deep Field South (CDFS, 0.22 deg2). We perform a number of diagnostic tests indicating that the remaining level of systematics is consistent with zero for the GEMS and GOODS data confirming the success of our PSF correction scheme. For the parallel data we detect a low level of remaining systematics which we interpret to be caused by a lack of sufficient dithering of the data. Combining our shear estimate of the GEMS and GOODS observations using 96 galaxies arcmin-2 with the photometric redshift catalogue of the GOODS-MUSIC sample, we determine a local single field estimate for the mass power spectrum normalisation Ï8=0.59+0.13-0.17(stat)±0.07(sys) (68% confidence assuming Gaussian sampling variance) at a fixed matter density Ωm=0.24 for a ÎCDM cosmology, where we marginalise over the uncertainty of the Hubble constant and the redshift distribution. This estimate agrees only marginally with the WMAP-3 result of Ï8=0.761+0.049-0.048 (Spergel et al. 2007) and is significantly below values found by recent ground-based surveys. From this discrepancy we conclude that the CDFS is subject to strong sampling variance with a significant under-density of compact foreground structures. This is consistent with a recent study by Phleps et al. (2007), who find a strong deficiency of red galaxies in this field. In a second step we perform a preliminary cosmological weak lensing analysis of the HST/COSMOS Survey (1.64 deg2). The significantly increased statistical accuracy reveals previously undetectable residual systematic errors indicated by a significant B-mode signal. So far we have not been able to unambiguously identify their origin, but note that similar indications for remaining systematics have been found in an independent analysis of the same data by Massey et al. (2007). Using only B-mode-free scales (>1' in the shear two-point correlation function), we find Ï8 = 0.71±0.09 (68% confidence) from COSMOS for a flat ÎCDM cosmology and fixed Ωm=0.24, where the error includes the uncertainties in the redshift distribution, the Hubble constant, and the shear calibration, as well as a Gaussian estimate for sampling variance. This result is in excellent agreement with the WMAP-3 constraints, but is significantly below the estimates found by Massey et al. (2007). In addition to the cosmological weak lensing analysis we present a reconstruction of the projected mass in the COSMOS field, as well as first results from a weak lensing analysis of the HST/STAGES Survey targeting the galaxy super-cluster Abell 901/902. Furthermore, we briefly summarise ACS studies of galaxy clusters, which make use of the developed data reduction and weak lensing pipeline
Weak lensing mass bias and the alignment of center proxies
Galaxy cluster masses derived from observations of weak lensing suffer from a
number of biases affecting the accuracy of mass-observable relations calibrated
from such observations. In particular, the choice of the cluster center plays a
prominent role in biasing inferred masses. In the past, empirical miscentring
distributions have been used to address this issue. Using hydro-dynamical
simulations, we aim to test the accuracy of weak lensing mass bias predictions
based on such miscentring distributions by comparing the results to mass biases
computed directly using intra-cluster medium (ICM)-based centers from the same
simulation. We construct models for fitting masses to both centered and
miscentered Navarro-Frenk-White profiles of reduced shear, and model the
resulting distributions of mass bias with normal and log-normal distributions.
We find that the standard approach of using miscentring distributions leads to
an over-estimation of cluster masses at levels of between 2\% and 6\% when
compared to the analysis in which actual simulated ICM centers are used, even
when the underlying miscentring distributions match in terms of the miscentring
amplitude. We find that neither log-normal nor normal distributions are
generally reliable for approximating the shapes of the mass bias distributions,
regardless of whether a centered or miscentered radial model is used.Comment: 15 pages, 9 figures, submitted to MNRA
The mass distribution in an assembling super galaxy group at
We present a weak gravitational lensing analysis of supergroup SG11201202,
consisting of four distinct X-ray-luminous groups, that will merge to form a
cluster comparable in mass to Coma at . These groups lie within a
projected separation of 1 to 4 Mpc and within km s and
form a unique protocluster to study the matter distribution in a coalescing
system.
Using high-resolution {\em HST}/ACS imaging, combined with an extensive
spectroscopic and imaging data set, we study the weak gravitational distortion
of background galaxy images by the matter distribution in the supergroup. We
compare the reconstructed projected density field with the distribution of
galaxies and hot X-ray emitting gas in the system and derive halo parameters
for the individual density peaks.
We show that the projected mass distribution closely follows the locations of
the X-ray peaks and associated brightest group galaxies. One of the groups that
lies at slightly lower redshift () than the other three groups
() is X-ray luminous, but is barely detected in the
gravitational lensing signal. The other three groups show a significant
detection (up to in mass), with velocity dispersions between
and km s and masses between
and , consistent with independent measurements. These groups are
associated with peaks in the galaxy and gas density in a relatively
straightforward manner. Since the groups show no visible signs of interaction,
this supports the picture that we are catching the groups before they merge
into a cluster.Comment: 10 pages, 10 figures, accepted for publication by Astronomy &
Astrophysic
Automated detection of galaxy-scale gravitational lenses in high resolution imaging data
Lens modeling is the key to successful and meaningful automated strong
galaxy-scale gravitational lens detection. We have implemented a lens-modeling
"robot" that treats every bright red galaxy (BRG) in a large imaging survey as
a potential gravitational lens system. Using a simple model optimized for
"typical" galaxy-scale lenses, we generate four assessments of model quality
that are used in an automated classification. The robot infers the lens
classification parameter H that a human would have assigned; the inference is
performed using a probability distribution generated from a human-classified
training set, including realistic simulated lenses and known false positives
drawn from the HST/EGS survey. We compute the expected purity, completeness and
rejection rate, and find that these can be optimized for a particular
application by changing the prior probability distribution for H, equivalent to
defining the robot's "character." Adopting a realistic prior based on the known
abundance of lenses, we find that a lens sample may be generated that is ~100%
pure, but only ~20% complete. This shortfall is due primarily to the
over-simplicity of the lens model. With a more optimistic robot, ~90%
completeness can be achieved while rejecting ~90% of the candidate objects. The
remaining candidates must be classified by human inspectors. We are able to
classify lens candidates by eye at a rate of a few seconds per system,
suggesting that a future 1000 square degree imaging survey containing 10^7
BRGs, and some 10^4 lenses, could be successfully, and reproducibly, searched
in a modest amount of time. [Abridged]Comment: 17 pages, 11 figures, submitted to Ap
Probing Galaxy Dark Matter Haloes in COSMOS with Weak Lensing Flexion
Current theories of structure formation predict specific density profiles of
galaxy dark matter haloes, and with weak gravitational lensing we can probe
these profiles on several scales. On small scales, higher-order shape
distortions known as flexion add significant detail to the weak lensing
measurements. We present here the first detection of a galaxy-galaxy flexion
signal in space-based data, obtained using a new Shapelets pipeline introduced
here. We combine this higher-order lensing signal with shear to constrain the
average density profile of the galaxy lenses in the Hubble Space Telescope
COSMOS survey. We also show that light from nearby bright objects can
significantly affect flexion measurements. After correcting for the influence
of lens light, we show that the inclusion of flexion provides tighter
constraints on density profiles than does shear alone. Finally we find an
average density profile consistent with an isothermal sphere.Comment: 14 pages, 14 figures. Accepted for publication in MNRA
Weak-lensing shear measurement with machine learning: teaching artificial neural networks about feature noise
Cosmic shear is a primary cosmological probe for several present and upcoming
surveys investigating dark matter and dark energy, such as Euclid or WFIRST.
The probe requires an extremely accurate measurement of the shapes of millions
of galaxies based on imaging data. Crucially, the shear measurement must
address and compensate for a range of interwoven nuisance effects related to
the instrument optics and detector, noise, unknown galaxy morphologies, colors,
blending of sources, and selection effects. This paper explores the use of
supervised machine learning (ML) as a tool to solve this inverse problem. We
present a simple architecture that learns to regress shear point estimates and
weights via shallow artificial neural networks. The networks are trained on
simulations of the forward observing process, and take combinations of moments
of the galaxy images as inputs. A challenging peculiarity of this ML
application is the combination of the noisiness of the input features and the
requirements on the accuracy of the inverse regression. To address this issue,
the proposed training algorithm minimizes bias over multiple realizations of
individual source galaxies, reducing the sensitivity to properties of the
overall sample of source galaxies. Importantly, an observational selection
function of these source galaxies can be straightforwardly taken into account
via the weights. We first introduce key aspects of our approach using toy-model
simulations, and then demonstrate its potential on images mimicking Euclid
data. Finally, we analyze images from the GREAT3 challenge, obtaining
competitively low shear biases despite the use of a simple training set. We
conclude that the further development of ML approaches is of high interest to
meet the stringent requirements on the shear measurement in current and future
surveys. A demonstration implementation of our technique is publicly available.Comment: 31 pages, 26 figures, minor changes to match the version published in
A&A, code available at https://astro.uni-bonn.de/~mtewes/ml-shear-meas
Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56
The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for
addressing outstanding issues in both galaxy evolution and fundamental physics.
We present a reconstruction of the mass distribution from both strong and weak
gravitational lensing data. Multi-color, high-resolution HST ACS images allow
detection of many more arc candidates than were previously known, especially
around the subcluster. Using the known redshift of one of the multiply imaged
systems, we determine the remaining source redshifts using the predictive power
of the strong lens model. Combining this information with shape measurements of
"weakly" lensed sources, we derive a high-resolution, absolutely-calibrated
mass map, using no assumptions regarding the physical properties of the
underlying cluster potential. This map provides the best available
quantification of the total mass of the central part of the cluster. We also
confirm the result from Clowe et al. (2004,2006a).Comment: Accepted for publication in ApJ; Version with full-resolution figures
available at
http://www.slac.stanford.edu/~marusa/Work/bradac_strong_weak_III.pd
The effect of the environment on the structure, morphology and star-formation history of intermediate-redshift galaxies
With the aim of understanding the effect of the environment on the star formation history and morphological transformation of galaxies, we present a detailed analysis of the colour, morphology and internal structure of cluster and field galaxies at 0.4â€zâ€0.8. We use {\em HST} data for over 500 galaxies from the ESO Distant Cluster Survey (EDisCS) to quantify how the galaxies' light distribution deviate from symmetric smooth profiles. We visually inspect the galaxies' images to identify the likely causes for such deviations. We find that the residual flux fraction (RFF), which measures the fractional contribution to the galaxy light of the residuals left after subtracting a symmetric and smooth model, is very sensitive to the degree of structural disturbance but not the causes of such disturbance. On the other hand, the asymmetry of these residuals (Ares) is more sensitive to the causes of the disturbance, with merging galaxies having the highest values of Ares. Using these quantitative parameters we find that, at a fixed morphology, cluster and field galaxies show statistically similar degrees of disturbance. However, there is a higher fraction of symmetric and passive spirals in the cluster than in the field. These galaxies have smoother light distributions than their star-forming counterparts. We also find that while almost all field and cluster S0s appear undisturbed, there is a relatively small population of star-forming S0s in clusters but not in the field. These findings are consistent with relatively gentle environmental processes acting on galaxies infalling onto clusters
Weak lensing from space: first cosmological constraints from three-point shear statistics
We use weak lensing data from the Hubble Space Telescope COSMOS survey to
measure the second- and third-moments of the cosmic shear field, estimated from
about 450,000 galaxies with average redshift ~ 1.3. We measure two- and
three-point shear statistics using a tree-code, dividing the signal in E, B and
mixed components. We present a detection of the third-order moment of the
aperture mass statistic and verify that the measurement is robust against
systematic errors caused by point spread function (PSF) residuals and by the
intrinsic alignments between galaxies. The amplitude of the measured
three-point cosmic shear signal is in very good agreement with the predictions
for a WMAP7 best-fit model, whereas the amplitudes of potential systematics are
consistent with zero. We make use of three sets of large Lambda CDM simulations
to test the accuracy of the cosmological predictions and to estimate the
influence of the cosmology-dependent covariance. We perform a likelihood
analysis using the measurement and find that the Omega_m-sigma_8 degeneracy
direction is well fitted by the relation: sigma_8
(Omega_m/0.30)^(0.49)=0.78+0.11/-0.26. We present the first measurement of a
more generalised three-point shear statistic and find a very good agreement
with the WMAP7 best-fit cosmology. The cosmological interpretation of this
measurement gives sigma_8 (Omega_m/0.30)^(0.46)=0.69 +0.08/-0.14. Furthermore,
the combined likelihood analysis of this measurement with the measurement of
the second order moment of the aperture mass improves the accuracy of the
cosmological constraints, showing the high potential of this combination of
measurements to infer cosmological constraints.Comment: 17 pages, 11 figures. MNRAS submitte
On the shear estimation bias induced by the spatial variation of colour across galaxy profiles
The spatial variation of the colour of a galaxy may introduce a bias in the
measurement of its shape if the PSF profile depends on wavelength. We study how
this bias depends on the properties of the PSF and the galaxies themselves. The
bias depends on the scales used to estimate the shape, which may be used to
optimise methods to reduce the bias. Here we develop a general approach to
quantify the bias. Although applicable to any weak lensing survey, we focus on
the implications for the ESA Euclid mission.
Based on our study of synthetic galaxies we find that the bias is a few times
10^-3 for a typical galaxy observed by Euclid. Consequently, it cannot be
neglected and needs to be accounted for. We demonstrate how one can do so using
spatially resolved observations of galaxies in two filters. We show that HST
observations in the F606W and F814W filters allow us to model and reduce the
bias by an order of magnitude, sufficient to meet Euclid's scientific
requirements. The precision of the correction is ultimately determined by the
number of galaxies for which spatially-resolved observations in at least two
filters are available. We use results from the Millennium Simulation to
demonstrate that archival HST data will be sufficient for the tomographic
cosmic shear analysis with the Euclid dataset.Comment: MNRAS submitted, 18 pages, 13 Figure
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